1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus.
2. Description of the Background Art
It has been customary with an image forming apparatus to entrust a serviceman with maintenance. Today, however, many ordinary users own image forming apparatuses because of the decreasing size and cost of the apparatuses. The spread of image forming apparatuses forces a single serviceman to deal with a prohibitive number of apparatuses, increasing the interval between a user's call for a serviceman and the arrival of the serviceman at the user's station. It is therefore difficult for a serviceman to maintain image forming apparatuses situated at users' stations one by one.
On the other hand, the expansion of international transport means and trade has accelerated overseas production and export of image forming apparatuses. A service system relating to image forming apparatuses greatly depends on the country. This, coupled with the fact that the interval between a user's call for a serviceman and the arrival of the serviceman depends on the distance, obstructs a timely service.
Under the above circumstances, maintenance by users will become predominant over maintenance by servicemen in the future. At the same time, replacement of defective parts will replace repair of defective parts, so even unskilled users can maintain image forming apparatuses by themselves. In this sense, there is an increasing demand for technologies that allow users to easily maintain the expected operations of image forming apparatuses.
Japanese Patent Laid-Open Publication No. 2000-75733, for example, discloses a process cartridge including a photoconductive element or image carrier, a charger, a cleaning device and so forth therein. The process cartridge is removably mounted to the body of an image forming apparatus and should only be bodily replaced in the event of a trouble or at the time of replacement. This can be done even by an ordinary user.
More specifically, the process cartridge, a photoconductive element and the case of a cleaning device, which rotatably supports the element, are constructed integrally with each other. The cleaning case is loaded with process means for executing image formation with the photoconductive element. The process means include a charge roller for uniformly charging the photoconductive element and a cleaning blade and a cleaning roller that cooperate to scrape off toner left on the element after image transfer to a paper-sheet or similar recording medium. Such process means are arranged around the photoconductive element.
Japanese Patent Laid-Open Publication Nos. 10-177286 and 11-295952 each disclose a recording system including two image stations. At each image station, a developing device, a writing device and drive means are supported by an apparatus body via positioning members and accurately positioned relative to each other thereby. Because the developing device is positioned relative to the apparatus body, it constitutes a reference position for the entire process devices. In this case, a photoconductive element (or photoconductive element unit) is not mounted to the apparatus body, but is mounted to the developing device. The photoconductive element is therefore positioned relative only to the developing device. The photoconductive element is removable from the developing device while the developing device is removable from the apparatus body. Further, the photoconductive element and cleaning means are constructed integrally with each other.
However, the problem with the process cartridge is that when only part of the cartridge, e.g., the photoconductive element or any one of image forming members fails, the cartridge must be bodily replaced. Discarding even usable members increases the running cost of the apparatus and is apt to obstruct the spread of image forming apparatuses.
In parallel with the increasing demand for an advanced image forming apparatus, loads on an image forming device are increasing. We conducted a series of researches to find that the demand increasing on the market aggravated loads on a photoconductive element, among others. This is generally ascribable to three different causes, which will be described hereinafter.
A first cause is a decrease in the diameter of a photoconductive drum, which is a specific form of a photoconductive element, essential for the miniaturization of an image forming apparatus. Specifically, when a photoconductive drum is reduced in diameter, the exhaustion of the drum for a single sheet is accelerated for preselected image forming conditions. For example, when the drum diameter is reduced from 120 mm to 40 mm, the drum must make three times greater number of rotations for a given image size. Consequently, the electrical exhaustion ascribable to, e.g., discharge and mechanical exhaustion ascribable to a cleaning blade are tripled. Miniaturization has proceeded with, e.g., the image forming means of a developing device to a certain degree, but not with a photoconductive drum from the above-described exhaustion standpoint.
A second cause is a decrease in the thickness of a photoconductor film essential for high image quality. Today, image quality is approaching one achievable with a silver halide sensitive type of film in order to cope with photographic images and graphic documents. A typical implementation for realizing such high image quality is increasing resolution. However, when it comes to an electrophotographic system, high resolution is not attainable without resorting to a thin photoconductor film. For example, in the case of a photoconductor chargeable to negative polarity, a carrier generated in a CGL (Charge Carrier Generation Layer) by exposure is transported to the surface of the photoconductor via a CTL (Charge Carrier Transport Layer) to thereby form a latent image if the CTL is thick, then the distance of migration of the carrier increases and causes the carrier to part due to electric repulsion. This prevents a latent image from being accurately formed in accordance with a signal and results in an image whose dots are dislocated.
The problem discussed above arises not only when electrophotographic resolution is increased from 600 dpi (dots per inch) to 1200 dpi, but also when it is desire to enhance image quality while maintaining the resolution of 600 dip in order to meet the current demand for high image quality. To solve the above problem, it is necessary to reduce the thickness of the photoconductor film and therefore the distance of migration of the carrier. However, the photoconductor film is shaved or otherwise exhausted every time an image is formed thereon. A thinner semiconductor film therefore is shorter in life, i.e., it withstands only a smaller number of times of image formation. Moreover, assume that the photoconductor film is scratched or otherwise damaged at the time of removal of a jamming sheet or due to a stapler left on a document. Then, the drum must be immediately replaced in order to maintain expected image equality.
A third cause is an increase in loads on the photoconductive drum ascribable to the trend toward color image formation. Today, color images are increasingly used because information printed thereon are easy to understand. A color image differs from a black-and-white image in that a photographic image or a graphic image occupying a broad area on a sheet is often output. In addition, a color image often includes a solid background area. As a result, an image area increases for one time of image formation and aggravates the exhaustion of the image forming means including the photoconductive drum.
An image forming apparatus with a revolver type developing device is conventional. This type of developing device includes a plurality of developing chambers that selectively face a photoconductive drum. The image forming apparatus with the revolver is extensively used because it forms a color image at relatively low cost with a small number of parts. However, a photoconductive element included in this type of apparatus exhausts several times more than each developing chamber because it is subject to a plurality of developing chambers.
The three causes described above will reduce the life of a photoconductive drum relative to the life of the other image forming means. While various studies are under way to enhance the durability and life of a photoconductive element, studies are also under way to enhance the durability and life of the other image forming means. There is a tendency that the life of a photoconductive element decreases relative to the life of the other image forming means. This tendency disturbs the balance between the photoconductive drum and the other image forming means in the process cartridge as to life. The problem with the process cartridge heretofore pointed out is that the cartridge must be replaced with priority given to image forming means having the shortest life. The problem becomes more serious with a decrease in the life of the photoconductive drum; even image forming means still usable must be discarded together with the photoconductive element. This increases the user's expenses, wastes the manufacturer's labor necessary for collection, and brings about environmental pollution.
Particularly, various technologies for extending the life of a developer have recently been reported in the imaging art in order to reduce toner filming and carrier exhaustion. This makes the life of the photoconductive element and that of the developing device unbalanced.
In light of the above, each image forming means included in an image forming apparatus should preferably be removable from the apparatus independently of the other image forming means. However, considering the future trend toward user-oriented maintenance, how simply the user can replace each image forming means is the problem. Further, the different image forming means should be mounted to or dismounted from each other at the time of replacement. It follows that the replacement must be accurate enough to protect image formation from adverse influence before and after replacement.
To solve the problems particular to a process cartridge, Japanese Patent Laid-Open Publication No. 62-17761 discloses a copier including an image carrier and a developing device removable from a copier body independently of each other. This configuration is directed toward user-oriented maintenance available with a low running cost. Such a means-by-means removal scheme, however, cannot clearly show the user unaccustomed to an image forming apparatus which part of the apparatus should be dismounted alone. Further, when one of the developing device and image carrier is dismounted from the copier body, it is likely that the other of them is dislocated. It is difficult for the user to accurately position the developing device and image carrier relative to each other. It is true that the developing device and image carrier are mounted to the copier body independently of each other, and therefore each means is positioned relative to the copier body. However, the relative position between the developing device and the image carrier is apt to vary before and after replacement and effect image quality. Relative position between the developing device and the image carrier is a decisive factor in the image quality aspect.
It is necessary with the copier taught in the above Laid-Open Publication No. 62-17761 to provide the individual part with accuracy high enough to insure accurate relative position between the developing device and the image carrier. This results in an increase in cost. This is also true with technologies proposed in Japanese Patent Laid-Open Publication No. 61-273559 and similar to the technology of the above document.
As stated above, conventional technologies are not user friendly and are apt to vary image quality before and after replacement.